After deleting ./moodledata/muc/* REDIS configuration is lost.
Start from a working REDIS setup
Verify the REDIS cache store is shown with a number >= 1 within the site administration
Delete the contents of ./moodledata/muc/
Go back to the site administration cache settings page and find the REDIS cache store is 0
If you had had set REDIS as cache store for "Application" and/or "Session" respective options will be reset to "Default file store for application caches" and to "Default file store for session caches" respectively (section "Stores used when no mapping is present").